Most people are familiar with air filters used in their cars. These filters are essential to proper operation of the engine, and help extend the life of the engine and its components. Automotive air filters must be replaced periodically because they become clogged and thus inhibit the flow of air into the engine. To the typical consumer, the air filter is cheap, and its replacement is a small additional bother that is handled along with oil changes. However, in dusty environments, high performance applications, industrial and farming applications, the cost of air filters and the burden of replacement may be significant, and a significant increase in filter performance and lifespan can be very valuable.
The air available to the typical automotive or industrial combustion engine always often carries some dirt and debris, or particulate material. Particulate material can cause substantial damage to the internal components of an internal combustion system if taken into the engine. The function of the air intake filter is to remove the particulate matter from the intake air, so that clean air is provided to the engine.
Choosing filter media that has a high filter efficiency, determined by the percentage of entrained material removed from the intake air, is important because any particulate matter passing through the filter will harm the engine. The choice of filter media which is permeable to air flow is important because the interposition of the filter into the intake air stream can impede air flow, and this decreases engine efficiency, horsepower, torque, and fuel economy. It is desirable, then, that an air filter effect both a minimal reduction in airflow as well as a minimal increase in the resistance, or restriction, to air flowing into the engine. The choice of filter media that can effectively filter air for extended periods without becoming clogged is also important, so that operation of the engine need not be interrupted frequently to change the air filter.
The features and filter design choices that lead to improvements in one of these parameters can lead to losses in the other performance parameters. Thus, filter design involves trade-offs among features achieving high filter efficiency, and features achieving a high filter capacity and concomitant long filter lifetime. As used herein, filter efficiency is the propensity of the filter media to trap, rather than pass, particulates. Filter capacity is typically defined according to a selected limiting pressure differential across the filter, typically resulting from loading by trapped particulates. For systems of equal efficiency, a longer filter lifetime is typically directly associated with higher capacity or depth loading, because the more efficiently a filter medium removes particles from a fluid stream, the more rapidly that filter medium approaches the pressure differential indicating the end of the filter medium life.
A particular filter medium can be very efficient, with a single layer removing a large percentage of the particles entrained in the fluid, for example, by collecting particles as a dust cake on the dirty side of the filter. Such “surface-loading” media includes paper and dense mats of cellulose fibers, with small pores. Initially, the dust cake can increase filter efficiency by itself operating as a filter. Over time, the dust cake tends to shorten the media lifetime, as more trapped particles occlude the filter medium surface pores, resulting in increased differential pressure across the filter. Depending upon the airflow through, and operating conditions of, the filter, a high-efficiency surface-loading filter medium can quickly reach a lifetime load. To extend filter lifetime, filter media can be pleated, providing greater filtering surface area.
Structural considerations are also a factor in filter design. If the filter is too flexible as with less restrictive media, the vibrations inherent in internal combustion engines may cause the filter to disengage from one or more areas of its housing thus allowing unfiltered air to bypass the filter. Many pleated filters require additional structural elements to create a filter with sufficient rigidity to retain engagement with the filter housing.
What is needed is a filter having good depth loading capacity with inherent rigidity to permit use of low restriction media while minimizing the likelihood of filter housing disengagement and filter bypass.